Insulin-induced in situ phosphorylation of the insulin receptor located in the plasma membrane versus endosomes.

The binding of insulin to the plasma membrane insulin receptor initiates two dynamic processes: (i) autophosphorylation of the receptor on tyrosine residues, activating the intrinsic tyrosine kinase activity required for insulin signaling, and (ii) endocytosis of the receptor. Recent evidence (Kublaoui et al., J. Biol. Chem. 270, 59-65, 1995) demonstrates that in stimulated adipocytes, the internalized insulin receptor is not only more highly phosphorylated than the receptor remaining on the plasma membrane of the cell, but that IRS-1 binding and phosphorylation also occur in the endosomes. These data suggest that the intracellular insulin receptor mediates insulin action. Utilizing 3T3-L1 adipocytes, we substantiate and extend these findings to document the distribution of receptor between the plasma membrane and the endosomal compartment of insulin-stimulated cells and map the extent and location of the tyrosine phosphorylation sites on the receptor residing in these two cellular locations. We find that following insulin stimulation (i) 90% of the receptor-associated phosphate is located in the endosomal compartment, (ii) the endosomal receptor is most highly phosphorylated in the tyrosine kinase domain, and (iii) significant levels of juxtamembrane domain phosphorylation are detected in the endosomal receptor. These data support the role of the endosomal insulin receptor as the major transducer of insulin action.